This invention generally relates to electro-initiation/detonation devices and methods for initiating the ignition of a pyrotechnic material. More specifically, this invention is particularly suitable for initiating the ignition of solid chemical composite propellants such as, but not limited to, initiating the ignition of boron mixed with potassium nitrate (B/KNO3) which, in turn, may be used to ignite the primary propellant of a rocket motor.
In order to ignite a pyrotechnic material such as, but not limited to, a solid propellant used in rockets, missiles, and launch vehicles, an initiator, containing a small quantity of confined pyrotechnic material, is typically used for initializing intermediate thermal energy positioning, boosting and heat spreading pyrotechnic structures, the functional conclusion of which leads to ignition of the primary pyrotechnic material in a satisfactory manor. As known within the art, an initiator, upon receiving a prescribed electrical, optical, thermal, or mechanical energy input, may be generally classified in one of two general types of initiators. The first type or style of initiator may, generally speaking, be referred to as a deflagratory type initiator in which the initial charge of pyrotechnic material burns in an extremely rapid and preferably extremely efficient manner but which does not burn in what could be characterized as an detonative, explosive, or violent manner. The second general type of initiator may generally be referred to as a detonative type or detonator in which the initial charge of pyrotechnic material explodes violently and often destructively. In some applications, an initiator or a detonator may work essentially equally as well for ignition of the main charge of pyrotechnic material as it may not be critical if the main charge is ignited in a detonative manner or if it is initiated in a relatively less violent, deflagratory manner. However, in other applications, it may be desired or required that the pyrotechnic material of the initiator burn extremely rapidly and efficiently, but do so without detonating or exploding. In yet other applications, it may be preferred or required that the initial charge of pyrotechnic material ignite in a detonative manner so as to properly trigger the main or primary charge, or alternatively a train of multiple charges of a given detonative device, system, or arrangement. Alternatively, an initiator or detonator, often referred to collectively as an ignition system by those skilled in the art, may be used to trigger a sequential ignition of a plurality of charges consisting of various pyrotechnic materials and configurations which ultimately ignites or detonates one or more main charges of pyrotechnic material. Such a preplanned, sequential ignition of one or more intermediate pyrotechnical charges is often referred to as a “pyrotechnic train,” or simply as a “train.”
Thus, when an initiator is used to ignite, for example, a solid propellant rocket or missile motor, the initiator, upon receiving a predetermined pulse of energy from an “arm and fire signal processing device,” will cause the pyrotechnic material contained within the initiator to combust. This event is typically not sufficiently energetic to ignite the solid propellant of the rocket motor directly without excessive uncertainty and, therefore, requires thermal power boosting through a pyrotechnic train, usually deflagratorily. The burning solid propellant will then provide a thrust profile, predictably positioned in time, designed to propel the missile, rocket, or vehicle along its planned trajectory. Upon the initiator receiving the input to fire the missile, the ignition process will typically occur in well under a second, with those skilled in the art measuring the ignition process in at least milliseconds and often in microseconds.
Regardless of whether a primary charge is to be ignited by one or more initial, or precursor charges in a degflagratory manner, or whether a primary charge is to be ignited by one or more initial or precursor charges in a detonative manner, the functional embodiment of the ignition initiator must be extremely resistant to accidental functioning. The initiator must be maximally immune to functioning by spurious electromagnetic energy and other signal and power sources indigenous to the military environment because of the unforgiving nature of pyrotechnic devices, which could inadvertently initiate or detonate the main or primary charge of pyrotechnic material.
The art is replete with a number of types of initiators and detonators, which are collectively referred to herein as initiators unless otherwise specified. The art is also replete with a wide number and variety of pyrotechnic materials used within initiators. Two of the most generally favored types of initiators are those which are actuated by electrical current, i.e., having electrical leads connected to an electrical resistance element that ignites the initiator pyrotechnic, and those which are optically actuated, i.e., having lasers or pumped laser-rods that project light through an optical fiber at an energy level sufficient to ignite the pyrotechnic material.
Representative and currently favored types of pyrotechnic materials used in initiators are boron potassium nitrate (B/KNO3), zirconium potassium perchlorate (Zr/KClO4) and nitrocellulose (abbreviated NC). Other fuels/oxidizers used in prior art initiators include magnesium (Mg), aluminum (Al), magnesium-polytetrafluoroetheylene (Mg-PTFE) and other metal-based fuels mixed with selected oxidants. Furthermore, it is common for small volumes of plasticizers and carbon black to be mixed into the pyrotechnic as needed for a particular application.
Solid propellants may include any of the above materials, as well as more conventional propellants such as smokeless powder, black powder, or more modern solid propellant grains known within the art including, but not limited to, nitrocellulose (NC), nitrocellulose/nitroglycerine (NC/C3H5(NO3)3), and ammonium perchlorate (NH4ClO4) based solid grain propellants.
It is well known in the art that a detonating or deflagrating ignition device or system utilizing an exploding bridge wire may be used to fire a rocket or a missile motor. Upon such a prior art bridgewire detonating device being ignited by an electrical current being applied to the resistive bridge wire, it rapidly heats to a temperature sufficiently high enough to explode and thereby ignite the pyrotechnic material. Although effective, such an activation scheme is prone to a number of problematic characteristics. One such potential problem is the accidental activation of the initiator by way of spurious or stray radio frequency interference (RFI), electromagnetic interference (EMI), electrostatic discharge (ESD), lightning effects (LE), electromagnetic pulse (EMP), power supply transients (PST), or other incidental and interfering energy being present, collectively referred to herein as spurious EMI, of such a magnitude that the initiator is inadvertently activated. Spurious EMI susceptibility may be quite serious whether in a commercial setting, such as where construction, demolition, or mining is taking place, or other areas in which spurious EMI activity could, or does, exist. Also, such spurious EMI susceptibility may not be tolerated in certain environments where a wide variety of spurious and EMI activity occurs as a matter of course.
Another characteristic of some prior art initiators that may be problematic is that the pyrotechnic material may be subject to unwanted “conditioning” upon being exposed to electrical current at a level too low to cause ignition. This conditioning may, in certain circumstances, unacceptably modify the future firing characteristics of the pyrotechnic material. That is, it is known that in some initiator arrangements, the pyrotechnic material or charge may be subjected to one or more exposures of electrical energy at a magnitude which do not cause ignition but, nonetheless, are of a magnitude which results in the pyrotechnic charge experiencing a noticeable change in its dielectric qualities. Thus, in order to actually ignite the charge after such exposure, an increased amount of current for an increased amount of time is required to successfully ignite the conditioned pyrotechnic charge as compared to a nonconditioned charge of the same pyrotechnic material. Thus, in some applications a more powerful and/or more complex ignition system may be required to ensure operational readiness as compared to what otherwise would be sufficient were the charge not subject to such unwanted conditioning.
A problem with prior art detonators which employ exploding bridge wires, is that such detonators are generally limited to ignition of pyrotechnic materials that are considered to be sensitive or relatively unstable. This is because it takes a relatively low amount of current to sufficiently energize a bridge wire to a high enough temperature to initiate the ignition of a sensitive material, but such a low (and reasonable) amount of current is not practical to adequately or reliably initiate the ignition of less sensitive pyrotechnic materials. That is, having a power supply robust and powerful enough to provide the requisite large amount of electrical current and voltage product needed to provide the high energy to function a suitable bridge wire structure often being used to detonate more stable, less sensitive pyrotechnic materials, may not be practical for initiating the ignition of solid propellant motors in air-launched missiles, easily movable and transportable ground-launched missiles, or other highly mobile launching venues and platforms.
Thus, in order for an ignition system (IS) to be of practical use regardless of the pyrotechnic material to be carried and energized within, the IS must be reliable and immune to accidental triggering so as not to create an undue or unwanted risk to personnel, yet not require a relatively large amount of electrical power that would not be practical or which would be unduly costly to have on hand. To elaborate, if an overly stable, or insensitive, pyrotechnic material is selected, the size, weight, and/or complexity of a power supply, even if augmented with capacitors and such, may not be powerful enough or operationally reliable enough to ignite the overly stable pyrotechnic material. Alternatively, even if an electrical power supply is made powerful and reliable enough, it is imperative that it not limit the practicality or versatility required for a given application such as being too heavy, too large, too complex or too fragile to be practical for initiating the ignition, for example, of a solid propellant missile motor.
One example of an ordnance initiation ignition system is disclosed in U.S. Pat. No. 5,144,893 issued to Zeman et al., which system incorporates a firing circuit connected to an electrical discharge initiation element in communication with a shock transmission tube that is, in turn, in communication with a pyrotechnic material such as B/KNO3. The initiation element is provided with a pair of electrical conductors, an insulator for separating the conductors, a cover to provide support and a protective environment in which the electrical conductors terminate into an air gap and a spark gap. Upon a sufficiently high voltage or current being applied to the conductors, a shockwave is produced that ruptures a thin cover or diaphragm of plastic or paper with the shock wave propagating through the shock transmission tube, which may be lined with a secondary explosive, ultimately reaching the pyrotechnic material and igniting it. Such an air gap and spark gap arrangement offers enhanced safety because it eliminates the need to incorporate a pyrotechnic-initiating material within the immediate vicinity of the electrical discharge initiation element. However, a gap arrangement requires ignition to be performed by way of a shockwave-induced detonation, which may not be suitable for many applications such as those where a deflagratory initiation is desired or required.
As a result of the desirability of using less sensitive pyrotechnic materials within ignition systems, laser-fired initiators have been extensively developed. An early example of a laser-fired initiator is disclosed in U.S. Pat. No. 3,408,937 issued to D. J. Lewis et al. As shown and described therein, a laser, such as a gaseous laser, a ruby-rod laser, a neodymium laser, a semiconducting laser, or a chemical laser, is combined with an optical fiber into which light energy is pumped or pulsed. The optical fiber is directed into a detonator/initiator containing a pyrotechnic material such as a detonative material including dynamite, cyclotrimethylenetrinitraime (RDX), trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), mercury fulminate or a deflagratory material such as black powder, metal oxides, metallized polyurethanes, various powdered fuels, high temp and carbon oxidant mixes. Because the laser output may generate enough energy to reach temperatures on the order of 2,000° C., laser-fired initiators were widely adapted as the temperatures were high enough to detonate less sensitive pyrotechnic materials. A further benefit offered by laser or laser-rod ignition systems is that optical fibers are immune to stray EMI/RMI as well as stray optical energy.
Another example of a laser-initiated ignition system is disclosed in U.S. Pat. No. 5,206,455 issued to Williams et al. The system disclosed in Williams et al. is described as being suitable for both deflagrating initiators and deflagration-to-detonation, devices and focuses on such systems being used in flight termination systems should a rocket exceed a predetermined range or should the rocket approach or threaten a safe-guarded area. The subject ignition system includes the use of laser energy to fire both a deflagrating initiator and deflagration-to-detonation device via fiber optic cable assemblies. In a preferred embodiment, six solid state lasers are used to provide the laser energy to ignite the various self destruct charges incorporated within the flight termination system. As can be appreciated upon reviewing the Williams et al. patent, the ignition system disclosed therein is quite elaborate and sophisticated in order to provide the amount of safety and redundancy that is absolutely required in the intended applications.
Notwithstanding the proven operational characteristics and reliability of ignition systems known within the art, there is a need for ignition systems that are relatively simple yet still offer the requisite reliability, an acceptable level of EMI/RFI immunity, operational efficiency, acceptable levels of safety, highly resistant to environmental contaminates and ambient temperatures, and are practical to fabricate and operate. Furthermore, there remains a need for ignition systems that are more economical to construct and maintain, and are versatile enough to use in a variety of pyrotechnic applications and not merely within a few specific, narrow applications. For example, laser-fired ignition systems often require an optical lens to be provided and sealed with respect to a container or column of a selected pyrotechnic material so that light exiting the optical fiber may be adequately focused on the material to ignite it. Such a sealed optical lens can be difficult to construct and care must be taken that all seals are strong enough to withstand the ultimate deflagration or detonation of the pyrotechnic material, such as in missile motor applications.
In many applications, such as when stable, less sensitive pyrotechnic materials are incorporated as a propellant in missile assemblies, such stable, less sensitive pyrotechnic materials allow for the installation of an initiator at the time the missile is assembled instead of just prior to when the missile is to be readied for firing. Thus, there remains a need for an electrically activated initiator that may be incorporated within an ignition system which is especially suitable for use in connection with stable, less sensitive pyrotechnic materials, such as B/KNO3 and Zr/KClO4 for example, which in turn, may be used to ignite the primary propellant of a solid propellant rocket motor.
Additionally, it would be beneficial to the art if an electrically activated deflagrating initiator were available featuring a pyrotechnic material container or body structure which is easily sealed with a robust, easy to construct hermetic seal made of readily available material. Such a seal would offer a further advantage over the hermetic seals, which are often used in connection with conventional laser-fired ignition systems, which seals tend to be relatively expensive and fairly difficult to construct due to the nature of the materials used. That is, seals used in connection with optical fibers are often quite elaborate as the optical fiber(s) used with laser-fired systems must allow optical energy to readily pass into the volume of pyrotechnic material to be ignited yet the hermetic seal must be able to prevent the high pressures generated by the ignition process from destroying the seal and possibly causing mechanical and operational problems in the proximity of the ignited pyrotechnic material.
A yet further need in the art is for an ignition system incorporating simplified secondary safety features enabling the ignition system to meet various standards, regulations, and other formal requirements and mandates that may be applicable to providing an acceptable safe arm/fire ignition system used in connection with the ignition of fuel/oxidants used in solid propellant missile and rocket motors. Such applicable standards include, for example, Military Standards published and made publicly available by the United States Department of Defense (U.S. DOD). One such publicly available standard is Military Standard 1901 (MIL-STD1901) which is entitled, “Munition Rocket and Missile Motor Ignition System Design, Safety Criteria For.” MIL-STD-1901 sets forth that an electrically-fired ignition system (IS), which contains only B/KNO3 meeting MIL-P-46994, or other appropriately qualified and tested pyrotechnic materials in lieu of or possibly in combination with B/KNO3, provided in the pyrotechnic train of the IS need not be provided with a mechanical interrupter (for example, a shutter, slider, or rotor for interrupting the pyrotechnic train). Thus, a particular ignition system incapable of being inadvertently initiated upon being subjected to an electrical potential of less than 500 volts but incorporating an electrical circuit that requires and can produce an initiation voltage preferably well in excess of the threshold 500 volts would be beneficial, if not required, for many applications so as to prevent unintentional arming and firing. Furthermore, an additional benefit of providing the industry with an electrically-activated arm/fire system which can be configured to conform with publicly available MIL-STD-1901 (Munition Rocket and Missile Motor Ignition System Design, Safety Criteria For), MIL-P-46994 (Pellets/Granules Boron/Potassium Nitrate), MIL-I-23659 (Initiators, Electric, General Design Specification For), and MIL-HDBK-1512 (Electroexplosive Subsystems, Electrically Initiated, Design Requirements and Test Methods) would, in certain applications, dispense with the added cost associated with providing an otherwise required pyrotechnic train interrupter within the design of such an electrically-fired arm/fire system.
It would further be desirable to have a small sized, laser-free and interrupter-free ignition system that could be easily tailored for use as either a bulk resistance initiator or as a dielectric breakdown initiator and which, upon being triggered, would have a minimal amount of ejecta so as to minimize the possibility of contaminating or interfering with the subsequent ignition of the pyrotechnic train or the primary charge or propellant. Furthermore, it would also be desirable to have an initiator which is intrinsically reliable, testable, relatively non-complex, exhibits a no-response immunity to voltages of less than 500 volts, and possesses a very high tolerance to temperature extremes, thermal and mechanical shock, drop testing, vibration, a sufficiently long storage life, and an acceptable post-trigger life expectancy.